The invention relates to an x-ray diagnostic installation for radiography and fluoroscopy, comprising an image intensifier-television chain and a circuit for the automatic influencing of at least one parameter determining the x-ray image quality, which circuit exhibits a radiation sensitive sensor, a setpoint value transmitter and a comparator for the comparison of an adjusted setpoint value with the actual value delivered by the sensor.
X-ray diagnostic installations of this type are known in which the circuit is a control loop for the dose rate for the purpose of maintaining the mean image (or picture) brightness or dose at a selected value during a given exposure time. In addition, the circuit can also be a disconnect circuit which effects the switching-off of the x-ray tube upon attainment of a specific radiation dose. As sensors, photomultipliers or ionometric measuring chambers for the measurement of the dose rate behind the patient are employed. In addition, the image pickup tube may also be employed as sensor (German Pat. No. 20 32 780). The obtained signal current is either employed in order to interrupt the x-radiation upon attainment of an adjusted, desired dose setpoint value, which effects a sufficient film density in the case of a picture record (image exposure) or to influence the radiation dose rate, by means of corresponding control of the electric values at the x-ray tube, and determining said radiation dose rate, in such a manner that, in the case of fluoroscopy, the mean image brightness remains constant, or, in the case of expiration of a fixed photographic exposure time, the desired dose per image is obtained.
In the case of exposure of x-ray films with an automatic exposure timer several difficulties arise. In the case of x-ray film photographs with large area film formats in front of the x-ray image intensifier, only an ionization chamber arranged in front of the image cassette supplies a correct exposure. However, due to the arrangement of this measuring chamber in front of the x-ray image intensifier, the distance between the radiation image plane and the radiography subject is increased. The enlargement scale is thereby raised, as a consequence of which blurring (or unsharpness) in the radiation image is undesirably increased.
Therefore, for the purpose of light brightness control of the fluoroscopy image, a photomultiplier is employed which is subjected to the light which is coupled out of the path of rays of the optical (or lens) system between the x-ray image intensifier and the television camera, or the television camera is employed as sensor. The dependency of the x-ray image intensifier upon the size of the image format here influences the control adjustment, so that defective images can result.
The contrast transfer factor, i.e. the conversion factor of the x-ray image intensifier, is dependent inter alia upon the electrode voltage determining the image format. In addition, the background of the x-ray image intensifier influences the measurement. With increasing size of the image format defined by the collimator on the inlet fluorescent screen, the luminescence per unit area also increases. As a consequence of this, upon altering the image format, the luminescence at the outlet screen of the x-ray image intensifier can vary to such an extent that likewise defective images can result.